Medical devices may be sterilized before use in order to minimize the likelihood that a device contaminated by, e.g., microorganisms might be used on a subject, which could cause an infection in the subject. Various sterilization techniques may be employed, using sterilants including one or a combination of steam, ethylene oxide, chlorine dioxide, ozone and hydrogen peroxide. Often the chemical sterilants are employed in a gaseous and/or a plasma form. For these techniques, sterilization is typically conducted within a sterilization chamber of a sterilization system. For certain chemical sterilization techniques, such as those using hydrogen peroxide, the sterilization chamber typically includes a vacuum chamber that is not only capable of achieving low pressures therein, but of also introducing sterilants therein and withdrawing sterilants therefrom. Some chemical sterilization processes, such as those that use ethylene oxide, require water vapor within the vacuum chamber to be effective. However, for other chemical sterilization processes, such as those that use hydrogen peroxide, water in vapor, liquid, or solid form within the vacuum chamber may decrease effectiveness.
A typical chemical vapor sterilization process for medical devices begins with medical-facility personnel preparing the devices for sterilization by washing the instruments with water and/or a washing solution to remove solids and liquids from the instrument. The personnel then dries the instruments, (e.g., using heat, medical-grade compressed air, and/or towels) and perhaps wraps them in a wrap suitable for sterilization, which acts as a barrier to microorganisms but that permits passage of a sterilant therethrough. Instruments wrapped in a wrap are sometimes referred to as a sterilization pack or load. The load is then placed into the vacuum chamber of the sterilization system and the chamber is closed (sealed), typically by closing the chamber's door. The chamber may be heated, which may help vaporize water that may be within the chamber. Next, the atmosphere in the chamber, which may include water vapor, is evacuated. In some sterilization procedures, air within the vacuum chamber may be excited to form an air plasma, which may further aid in vaporizing water for removal from the chamber. After achieving a low pressure, sometimes referred to as a vacuum or a rough vacuum, a sterilant is introduced into the chamber, either in gaseous form or as a mist that vaporizes in the low pressure environment of the chamber. The added gas in the chamber slightly raises the pressure in the chamber. The sterilant spreads quickly throughout the chamber, entering small or confined spaces, such as cracks, crevices, and lumens in the medical devices contained therein. The sterilant bathes the medical devices, which kills bacteria, viruses, and spores disposed upon and within the devices that it contacts. In some sterilization procedures, particularly low-temperature procedures that utilize hydrogen peroxide, the hydrogen peroxide gas may be excited via an electric field to change the gas into a plasma. Finally, the sterilant is evacuated from the chamber and the chamber is returned to the ambient pressure. After the sterilization process has ended, the instruments may be removed from the chamber.
Typically, healthcare personnel check whether the sterilization process was efficacious using various techniques known in the art, e.g., by use of a self-contained biological sterilization indicator, such as the STERRAD® CYCLESURE® 24 Biological Indicator, manufactured by Advanced Sterilization Products, Division of Ethicon US, LLC, located in Irvine, Calif. Confirmation using this biological indicator typically requires about twenty-four hours. During this time, while the effectiveness of the sterilization remains unconfirmed, medical personnel may decide not use the medical devices. This may cause inventory management inefficiencies for a health care provider, such as a hospital, because, for example, the medical devices should be stored while they cannot be used, perhaps requiring the health care provider to keep more medical devices in its inventory than it otherwise would to ensure a sufficient supply of medical devices. Alternatively, health care providers may use the medical devices before the sterilization confirmation is completed and sterilization efficacy confirmed. However, using the medical devices before sterilization efficacy has been confirmed may expose a subject of a medical procedure to risk of infection from the medical devices. Given the total amount of time medical devices may be unsuitable for use because of the time required to conduct a sterilization process and the time required to confirm that the sterilization process was efficacious, healthcare personnel desire updated sterilization processes and confirmation techniques that require less time to conduct and reduce the likelihood that a process may fail as compared to those presently available.
An example of a commercially available sterilization chamber is the STERRAD® 100NX® System manufactured by Advanced Sterilization Products, Division of Ethicon US, LLC, located in Irvine, Calif. The 100NX® is advertised as being capable of sterilizing most general surgical instruments in 47 minutes. The cycle temperature of the 100NX® is advertised as being between 47° C. to 56° C. These temperatures are preferred for helping to vaporize residual water with heat without over-heating the instrument, which could compromise the function or structure of instruments. Further, these temperatures are preferred for generating plasma, which helps improve the effectiveness of the sterilization process and further helps vaporize any residual water, and to aid in removing hydrogen peroxide from the vacuum chamber.
Commercially available sterilization systems that employ, e.g., hydrogen peroxide are designed to preferably operate without any water in their sterilization chambers. If healthcare personnel erroneously introduced water into the chamber, the water will begin evaporating as the pressure within the chamber is lowered to maintain a surface-pressure equilibrium between the water and its surroundings. This pressure equilibrium, which is also a function of temperature, is typically referred to as the vapor pressure of water. At 100° C., the vapor pressure of water is one atmosphere, or 760 torr, which is why it is commonly stated that water boils at 100° C. However, when the local pressure around water is less than 760 torr, the liquid water may change phase to water vapor at lower temperatures.
Latent heat is required for water to change phase to vapor. The evaporating water may draw at least some of this energy from remaining water, which lowers the temperature of the remaining water. As the pressure in the chamber continues to drop, and as evaporating water continues to lower the temperature of the remaining water, the pressure and temperature approach what is often referred to as the “triple-point” of water, i.e., the temperature and pressure combination at which ice, water, and water vapor exist in equilibrium. The triple-point temperature of water is 0.01° C. and the triple-point pressure of water is 4.58 torr. As the temperature and pressure approach the triple-point, the likelihood of ice-crystals forming within the remaining water increases.
Ice may inhibit contact of a sterilant with at least a portion of a medical device or instrument, including by potentially blocking lumens of the device. Accordingly, ice may cause a sterilization process to be inefficacious, which may lead to use of a non-sterile device on a subject or cause a hospital to subject the device to another round of sterilization, which requires additional valuable time. Moreover, sterilant may condense upon or become trapped within the ice, which could lead to chemical burns on the skin of medical personnel.
In addition to sterility itself, time and efficiency associated with sterilization processes for medical devices are important considerations for health care facilities. For example, hospitals often prefer to maximize the number of times a device may be used within a given time span, e.g., per week. Subjecting a wet medical device to a sterilization process thus not only increases the likelihood that the sterilization process will not be efficacious, it also wastes time and may lower the number of times per week that a device may be reused. Accordingly, medical personnel should remove all water from the medical devices after they have been cleaned but before they are placed into the sterilization chamber, or at least before sterilant gas is introduced into the vacuum chamber.
Some sterilization systems check for the presence of water in the sterilization chamber before they introduce a sterilant gas therein by checking for small increases in pressure inside the chamber while vacuum is being drawn. If no water is present in the chamber while vacuum is being drawn, the pressure decreases asymptotically without any increases therein. However, if any water is in the chamber while vacuum is being drawn, at least some of the water may turn to vapor, which may cause slight local increases in pressure. Accordingly, detection of a small pressure increase while vacuum is being drawn indicates the presence of water in the vacuum chamber. When water is detected, the sterilization process may be aborted so that excess water may be removed from the medical devices before attempting sterilization again. Aborting a sterilization process as soon as water is detected may help save time and resources as compared to continuing a sterilization process that may not be efficacious, and may help avoid use of a non-sterile device.
In some instances, instead of aborting the sterilization process, it may be preferable to attempt to remove the water from the vacuum chamber by a process called “load conditioning.” Load conditioning is typically accomplished by, first, some combination of heating and/or introducing plasma into the sterilization chamber and re-pressurizing the sterilization chamber to transfer energy to the water (or ice), and, second, drawing a vacuum anew to convert the water to vapor. Load conditioning may occur before, after, or both before and after vacuum is drawn in the chamber. In some instances load conditioning cannot remove water from the chamber. In other instances load conditioning may remove some but not all of the water. In such instances, additional load conditioning may be attempted, but doing so requires additional time and resources. Accordingly, where load conditioning cannot remove water from the chamber or where repeated attempts may be required to remove water, it may be desirable to forego load conditioning in favor of aborting the process so that excess water may be removed from the medical devices before attempting a new sterilization process.